In an optical fiber transmission system, an influence of chromatic dispersion in an optical fiber transmission line is inevitable. Accordingly, means to measure an amount of chromatic dispersion in an optical transmission line are demanded. Well-known means are disclosed in the following three references: (1) A. Sano et al., “Automatic dispersion equalization by monitoring extracted-clock power level in a 40-Gbit/s, 200-km transmission line,” Tu3.5, Vol. 2, pp. 207–210, ECOC'96; (2) M. N. Peterson et al., “Dispersion monitoring and compensation using single in-band Subcarrier tone,” WH4, OFC2001; and (3) Q. Yu et al., “Chromatic Dispersion Monitoring Technique Using Sideband Optical Filtering and Clock Phase-Shift Detection,” Journal of lightwave Technology, Vol. 20, No. 12, pp. 2267–2271, 2002.
In the first method described in the above reference (1), an optical signal is converted into an electric signal, and from the electric signal, electrospectral intensity to be affected by chromatic dispersion is detected. The electrospectral intensity decreases according to an increase of an amount of chromatic dispersion in an optical transmission line. This method estimates an amount of chromatic dispersion in an optical transmission line using such relation between the electrospectral intensity and the amount of chromatic dispersion.
In the second method described in the second reference (2), an optical transmitter transmits to an optical transmission line an optical signal superimposed by a tone signal of frequency f (Hz). In an optical receiver, the optical signal from the optical transmission line is converted into an electric signal, and a component of the tone frequency f (Hz) is extracted from the electric signal. Amplitude of the component of the tone frequency detected in the optical receiver decreases according to an increase of an amount of chromatic dispersion in the optical transmission line. The amount of chromatic dispersion of the optical transmission line is estimated using such relation.
In the third method described in the third reference (3), an optical transmitter generates and transmits to an optical transmission line an optical signal intensity-modulated by a test data (e.g. random data). In an optical receiver, a sideband component on the long wavelength side, that is a lower sideband component, and a sideband component on the short wavelength side, that is an upper sideband component, of intensity modulation are extracted from the optical signal input from the optical transmission line. Each sideband component is converted into an electric signal to extract a clock component. Phases of the two clock components are compared. This method uses such mechanism that a phase difference between two clocks depends on detuning amounts of two filters for extracting sidebands and on chromatic dispersion of a signal wavelength.
Measured results of the above-stated first and second conventional methods are, however, affected by factors other than the amount of chromatic dispersion, such as polarization mode dispersion (PMD) and an optical signal to noise ratio (OSNR) and therefore it is difficult to obtain an accurate amount of chromatic dispersion. Furthermore, in a configuration in which a PMD compensator is added, it is difficult to automatize the measurement of the amount of chromatic dispersion.
In the second conventional method, since it is necessary to dispose additional apparatuses on both transmitter and receiver sides, there is a disadvantage that the configurations of an optical transmitter and an optical receiver become complicated.
Although the third conventional method has an advantage of low dependency on PMD and OSNR, it is necessary to dispose two high-speed optoelectric converters, two clock extractors, and a phase comparator to compare a phase of outputs from the two clock extractors and accordingly the configuration becomes expensive and large-sized.